Reverse Osmosis (RO) Water Filters

PentekRO-2550 Reverse Osmosis System

Reverse osmosis, also known as hyperfiltration, is the finest filtration available today. It is the most common treatment technology used by premium bottled water companies. It is effective in eliminating or substantially reducing a very wide array of contaminants, and of all technologies used to treat drinking water in residential applications, it has the greatest range of contaminant removal. Reverse osmosis will allow the removal of particles as small as individual ions. The pores in a reverse osmosis membrane are only approximately 0.0005 micron in size (bacteria are 0.2 to 1 micron & viruses are 0.02 to 0.4 microns).

There are two types of reverse osmosis membranes commonly used in home water purification products: Thin Film Composite (TFC) and Cellulose Triacetate (CTA). TFC membranes have considerably higher rejection rates (they will filter out more contaminants) than a CTA membrane, however, they are more susceptible to degradation by chlorine. This is one of the reasons why it is important that a reverse osmosis system include quality activated carbon pre-filters.

A typical RO system is composed of an array of granular activated carbon (GAC) pre-filters, the reverse osmosis membrane, a storage tank, and a faucet to deliver the purified water to your countertop. Reverse osmosis systems vary in membrane quality, output capacity, and storage capacity.

How Reverse Osmosis Works

Reverse osmosis uses a semi-permeable membrane, allowing pure water to pass through it, while rejecting the contaminants that are too large to pass through the tiny pores in the membrane. Quality reverse osmosis systems use a process known as crossflow to allow the membrane to continually clean itself. As some of the fluid passes through the membrane the rest continues downstream, sweeping the rejected contaminants away from the membrane and down the drain. The process of reverse osmosis requires a driving force to push the fluid through the membrane (the pressure provided by a standard residential water system is sufficient - 40 psi+).

Applications

Reverse osmosis is an excellent choice for almost all home water purification needs. It is the most recommended solution for individuals on a pre-treated municipal water system. While reverse osmosis can be very effective in removing bacteria and viruses, it is not recommended that reverse osmosis be the only level of purification for water that contains or may contain biological contaminants (untreated well or lake water, for instance). For these applications consider a combined reverse osmosis / ultraviolet system or the addition of a complementary whole-house ultraviolet system for maximum effectiveness and protection against bacteria and viruses. Since membranes are subject to degrading by chlorine, iron, manganese, and hydrogen sulfide, and to bacterial attack, a sediment pre-filter and an activated carbon pre-filter and/or post-filter should be included with your reverse osmosis system. Water softeners can be used in advance of the RO system when household water is excessively hard to prevent pre-filter and membrane fouling. RO systems are generally the best choice for water contaminated with high nitrite levels as might be found in agricultural areas.

What Contaminants Does Reverse Osmosis Remove?

Reverse osmosis (RO) units remove substantial amounts of most inorganic chemicals (such as salts, metals, minerals) most microorganisms including cryptosporidium and giardia, and most (but not all) inorganic contaminants.

Reverse osmosis successfully treats water with dissolved minerals and metals such as aluminum, arsenic, barium, cadmium, chloride, chromium, copper, fluoride, magnesium, iron, lead, manganese, mercury, nitrate, selenium, silver, sulfate, and zinc. RO is also effective with asbestos, many taste, color and odor-producing chemicals, particulates, total dissolved solids, turbidity, and radium. When using appropriate activated carbon pre-filtering (commonly included with most RO systems), additional treatment can also be provided for "volatile" contaminants (VOCs) such as benzene, MTBE, trichloroethylene, trihalomethanes, and radon. Essentially, reverse osmosis is capable of rejecting bacteria, salts, sugars, proteins, particles, dyes, heavy metals, chlorine and related by-products, and other contaminants that have a molecular weight of greater than 150-250 daltons. The separation of ions with reverse osmosis is aided by charged particles. This means that dissolved ions that carry a charge, such as salts, are more likely to be rejected by the membrane than those that are not charged, such as organics. The larger the charge and the larger the particle, the more likely it will be rejected.

Arsenic

BacteriaandViruses

Bad Tastes& Odors

Chlorine

Fluoride

HydrogenSulfide

Heavy Metals

Nitrates

Radon

Sediment

Iron

VOC's

*

= Effectively Removes = Significantly Reduces = Minimal or No Removal

* Even though reverse osmosis is effective in removing bacteria and viruses, it is not recommended that you rely solely upon reverse osmosis if your water is contaminated with bacteria or viruses. Ultraviolet (UV) purification is also recommended.

For more specific information on contaminant removal by UV disinfection and a comparison to other filter / purification technologies, please CLICK HERE.

Advantages

very high rejection rate for a wide array of contaminants

very cost effective in the long term, costing as little as 5 cents per gallon of pure water once maintenance and water costs are factored in

Disadvantages

requires sediment and carbon pre-filtration (generally included as part of the system) to prevent membrane fouling

because reverse osmosis works against standard osmotic pressure, the process is generally fairly slow, producing roughly 15 gallons of purified water per day, and may require from 3 to 10 gallons of untreated water to make a single gallon of purified water

To reduce membrane fouling and to maintain optimal performance, it is important to ensure that scheduled replacements of carbon or other pre-filters are followed. The reverse osmosis membrane (thin film composite - TFC) should be replaced every 2-3 years.

Frequently Asked Questions About Reverse Osmosis

1. What is Reverse Osmosis?Reverse osmosis, also known as hyperfiltration, is the finest filtration available today. It is the most common treatment technology used by premium bottled water companies.

2. How does Reverse Osmosis work?Reverse osmosis uses a semi-permeable membrane, allowing pure water to pass through it while rejecting the contaminants that are too large to pass through the tiny pores in the membrane.

3. Can Reverse Osmosis be used on well water or water from other untreated sources (lake or river)?Yes, RO is generally an excellent choice for homeowners with well water. However, it is important to note that reverse osmosis does not provide foolproof protection against all microorganisms. You should have your water tested for bacteria, virus, and cyst contamination before relying solely on reverse osmosis. If microbiological contamination is present or suspected, you should combine reverse osmosis with an ultraviolet system for maximum effectiveness and protection against bacteria and viruses. A water softener or whole-house iron filter may also be advisable (depending on the level of relevant contaminants in your well water) to prevent membrane fouling, thereby ensuring maximum membrane life and effectiveness.

4. How often does the reverse osmosis membrane need to be replaced?With proper maintenance of your sediment and activated carbon pre-filters, your reverse osmosis membrane should last 2-3 years.

5. Why are reverse osmosis systems always combined with carbon and sediment pre-filters?The only major category of contaminants that reverse osmosis is not highly effective in removing (organic compounds) is specifically targeted by activated carbon filters. Pre-filters also prevent the reverse osmosis membrane from being fouled or clogged by sediment, chlorine, and other contaminants, thereby enhancing its effectiveness and lifespan.

6. Are all reverse osmosis systems equally effective?Absolutely not. Like all water filters, the effectiveness of a reverse osmosis system depends greatly on the quality of its components - especially its pre-filter cartridges and the membrane itself. Lower quality pre-filters will suffer from premature membrane fouling, as well as reduced performance, purified water output, and membrane life. TFC membranes are also much more effective than CTA membranes.

7. I notice that a reverse osmosis system will remove just about everything from my water, including some nutrients that are good for the body. Should I take a supplement to counteract the nutrients that I will no longer get through my water?No, this is not necessary. You should already be getting all of the nutrients, such as essential salts, vitamins, and other trace minerals from the food you eat and the other beverages you drink.

8. What is "crossflow"?Quality reverse osmosis systems use a process known as crossflow to allow the membrane to continually clean itself. As some of the fluid passes through the membrane the rest continues downstream, sweeping the rejected contaminants away from the membrane and down the drain. This prevents contaminants from backing up against the membrane and clogging it.

9. How much filtered water can a home RO system produce?Our RO systems produce up to 50 gallons of purified water per day but can be upgraded to 100 gallons per day if desired (at extra cost). If you opt to upgrade your membrane, you may need to also upgrade the storage tank to gain the benefit of the larger membrane. The actual amount of water produced in your home will depend on your household water pressure. Because reverse osmosis water purification occurs slowly (it is a very fine filter!), a storage tank is used to hold 3 gallons of purified water at all times so pure water is always at your fingertips.